In an oil refinery, the desalting of crude oil has been practiced for many years. The crude is usually contaminated from several sources, including, but not necessarily limited to:                Brine contamination in the crude oil as a result of the brine associated with the oil in the ground;        Minerals, clay, silt, and sand from the formation around the oil well bore;        Metals including calcium, zinc, silicon, nickel, sodium, potassium, etc.;        Nitrogen-containing compounds such as amines used to scrub H2S from refinery gas streams in amine units, or from amines used as neutralizers in crude unit overhead systems, and also from H2S scavengers used in the oilfield; and        Iron sulfides and iron oxides resulting from pipeline and vessel corrosion during production, transport, and storage.        
Desalting is necessary prior to further processing to remove these salts and other inorganic materials that would otherwise cause fouling and deposits in downstream heat exchanger equipment and/or form corrosive salts detrimental to crude oil processing equipment. Further, these metals can act as poisons for the catalysts used in downstream refinery units. Effective crude oil desalting can help minimize the effects of these contaminants on the crude unit and downstream operations. Proper desalter operations provide the following benefits to the refiner:                Reduced crude unit corrosion.        Reduced crude preheat system fouling.        Reduced potential for distillation column damage.        Reduced energy costs.        Reduced downstream process and product contamination.        
Desalting is the resolution of the natural emulsion of water that accompanies the crude oil by creating another emulsion in which about 2 to about 10 wt % percent relative wash water is dispersed into the oil using a mix valve. For lighter crudes, the wash water proportion may range from about 3 to about 5 wt %; for heavier (lower gravity) crudes, the wash water proportion may range from about 5 to about 8 wt %. The emulsion mix is directed into a desalter vessel containing a parallel series of electrically charged plates. Under this arrangement, the oil and water emulsion is exposed to the applied electrical field. An induced dipole is formed on each water droplet within the emulsion that causes electrostatic attraction and coalescence of the water droplets into larger and larger droplets. Eventually, the emulsion resolves into two separate phases—the oil phase (top layer) and the water phase (bottom layer). The streams of desalted crude oil and effluent water are separately discharged from the desalter.
The entire desalting process is a continuous flow procedure as opposed to a batch process. Normally, chemical additives are injected before the mix valve to help resolve the oil/water emulsion in addition to the use of electrostatic coalescence, although some additives or portions of additives may be injected elsewhere. These additives effectively allow small water droplets to more easily coalesce by lowering the oil/water interfacial tension.
Crude oil that contains a high percent of particulate solids can complicate the desalting process. The particulate solids, by nature, would prefer to transfer to the water phase. However, much of the solids in a crude oil from a field exists in tight water-in-oil emulsions. That is, oil-wetted solids in high concentration in the crude may help form tight oil and water emulsions that are difficult to resolve. These tight emulsions are often referred to as “rag” and may exist as a layer between the separated oil and water phases. The rag layer inside the desalter vessel may grow to such an extent that some of it will be inadvertently discharged with the water phase. This is a problem for the waste water treatment plant since the rag layer still contains a high percentage of unresolved emulsified oil.
As mentioned, much of the solids encountered during crude oil desalting consists of iron, most commonly as particulate iron such as iron oxide, iron sulfide, etc. Other metals that are desirably removed include, but are not necessarily limited to, calcium, zinc, silicon, nickel, sodium, potassium, and the like, and typically a number of these metals are present. Some of the metals may be present in a soluble form. The metals may be present in inorganic or organic forms. In addition to complicating the desalter operation, iron and other metals are of particular concern to further downstream processing. This includes the coking operation since iron and other metals remaining in the processed hydrocarbon yields a lower grade of coke. Removing the metals from the crude oil early in the hydrocarbon processing stages is desired to eventually yield high quality coke as well as to limit corrosion and fouling processing problems.
Several treatment approaches have been made to reduce total metal levels and these all center on the removal of metals at the desalter unit. Normally, the desalter only removes water soluble inorganic salts such as sodium or potassium chlorides. Some crude oils contain water insoluble metal organic acid salts such as iron naphthenate and calcium naphthenate, which are soluble or dispersed as fine particulate matter in the oil but not in water.
It would thus be desirable to develop a composition and method employing it that would cause most or all of the metals in the crude oil to transfer from the oil phase to the aqueous phase in a desalter operation, with little or no oil carry-under in the aqueous phase.